1. Field of the Invention
The present invention relates to an optical pickup for optically reading/writing information from/on an optical recording medium, and particularly to an optical pickup for use with disks each having a different thickness.
2. Description of the Related Art
A compact disk (CD), which is widely known as an optical recording medium, has a substrate thickness of 1.2 mm. As a recording layer, it includes a reflection surface for reflecting optical light, and the information on sounds, characters, graphics, etc. is stored in a number of pits formed in the reflection surface. Recently, a digital video disk (DAD) having a substrate thickness of 0.6 mm, which is capable of storing a large quantity of video information at a high density has been developed. An optical pickup for a DAD uses an optical light having short wavelength and collects light by an objective lens having a large numerical aperture (NA), thereby forming a small spot for reproduction at a high density. In the case of reproducing information stored in a CD by the optical pickup for a DAD, the reproduction is actually difficult, as the spherical aberration of the recording surface increases due to the difference in thicknesses of the substrates, and thus, the reproducing signals are degraded.
Conventionally, several kinds of optical pickups for use with disks each having a different thickness, have been supplied in compliance with the users' request. The conventional optical pickup for use with disks each having a different thickness, use a light source of a short wave length of 650 nm, and has a device for adjusting the numerical aperture of the objective lens according to the thickness of disk whose information is to be reproduced. In other words, when reproducing information stored in a DAD, a small spot is formed using the numerical aperture of 0.6 for the objective lens. When reproducing information stored in a CD, the spherical aberration is corrected by adjusting the numerical aperture to 0.45 using a part of the objective lens having a small numerical aperture, i.e., a part nearby the axis, or by restricting the beam diameter of the light incident upon the objective lens from the light source.
Nowadays, as a recording medium capable of reproducing information which is already recorded once, a CD-R is practically used. Moreover, the CD-R is required to be compatible with the aforesaid CD and DAD. For reference, the thickness of a CD-R is m, which is the same as the CD. Its recording layer is made of a phase-variable material in which the reflexibility of the recording wavelength and the reproducing wavelength are changed in a specific wavelength band. Also, the information stored in a CD-R can be reproduced in an optical pickup environment, similar to the environment where the information stored in a CD is reproduced.
However, as the conventional environment of the optical pickup for using disks each having a different thickness adopts a light source of short wavelength, it is not proper to reproduce the information stored in the CD-R.
Additionally, in the conventional optical pickup, as the optical environment conforms with the thin DAD, the information stored in the CD cannot be reproduced under optimum conditions.